Rotor unit and electric motor

ABSTRACT

A rotor unit for a brushless electric motor, incudes an annular rotor core surrounding a central axis, a plurality of magnet arrangements that are arranged around the rotor core in a circumferential direction of the rotor and that each have a convex outer peripheral face, an inner contact face, two axial end faces and two side faces pointing in the circumferential direction, a magnet holder having a number of holding portions which are each arranged between two adjacent magnet arrangements and are moulded onto a ring portion of the magnet holder. The holding portions are configured T-shaped in a cross section along a plane running transversely to the central axis and each have a shank portion and a head portion. The shank portion lies on the side faces of the magnet arrangements in a contact region and the head portion lies on the peripheral faces of the magnet arrangements.

The present invention relates to a rotor unit for a brushless electricmotor with the features of the preamble of claim 1 and to a brushlesselectric motor.

Electric motors in which the rotor has a permanent magnet are known fromthe prior art. The permanent magnets are arranged about a rotor core andlie on the outside thereof. The rotor defines the geometric axes and thedirections that should be used in this description and the claims too. Acentral axis coincides with the symmetrical axis of the rotor and alsorepresents the rotational axis of the rotor in the electric motor. Theaxial direction of the arrangement runs in the direction of therotational axis. The radial direction is characterised by an increasingdistance from the central axis. The permanent magnets on the rotor aretherefore on the outside in a radial direction. The peripheral directionin which each direction vector is aligned vertically to a radius of thearrangement runs tangentially to the rotor.

According to the prior art, the electric motor also has a statorarranged in a radial direction outside of the rotor, which statorsurrounds the rotor on the outside in a ring shape. The stator containsa number of electromagnets, which are generally formed from an iron coreand a coil. A suitable current supply to the windings of the statorgenerates a rotational field which accordingly generates a torque in therotor. The stator is arranged in a motor housing in which the rotor withits motor shaft is rotatably mounted.

The permanent magnets are usually made from a brittle material. Themagnets are not screwed to the rotor core but rather sit on end faces ofthe rotor core which point in an outwards direction, where they aremechanically held by a magnet holder. The magnet holder absorbs thecentrifugal forces that act on the magnets when the rotor rotates.

The magnet holders therefore have the object of holding the magnets inthe intended position in a fixed and precise manner. They are also usedas a guide. During manufacturing, the rotor core is initially equippedwith the magnet holder and the magnets are then inserted into theintended positions, wherein they are inserted in an axial directionalong the planar outer face of the rotor core between two respectiveholding portions of the magnet holder.

Various solutions for this are known from the prior art. Patentspecification U.S. Pat. No. 7,687,957 B2 discloses a rotor unit havingmagnet holders with linear contact faces for the magnets in an axialdirection. These linear contact faces have a low friction against thesurface of the magnets when the magnets are mounted. Their function isalso to keep the magnets in the intended position using spring force dueto their elasticity.

Patent application US 2015/0001978 A1, on the other hand, discloses arotor with magnet holders that are designed in a T-shaped manner in thecross-portion and sit with a shank portion between the magnets, while ahead portion of the T-shaped cross-portion pushes on the magnets fromthe outside in a radial direction, thereby securing the magnets in theirposition. The full surface of the head portion lies on the magnets. Inthis way, the fixing of the magnets is particularly secure. However, thefriction when the magnets are inserted between the magnet holders isundesirably high. This can cause wear or damage to the magnets.

The object of the present invention is therefore to create a rotor unitand an electric motor in which novel magnet holders on the one handposition the magnets sufficiently tightly and accurately on the rotorcore and on the other enable decreased friction between the magnetholders and the outer face of the rotor core on insertion of themagnets.

This object is achieved by a rotor unit with the features of claim 1 andan electric motor with a rotor unit of this type.

Since in the case of a rotor unit according to the invention theintention is for the contact region of the shank portion in a radialdirection of the side faces to have an extension of 30% to 70%, inparticular 40% to 60% of the radial extension of the side faces, themounting of the magnet arrangements is simplified as the frictionbetween the magnet holders and the magnet arrangements is decreased whenthe magnet arrangements are inserted in an axial direction.

The friction when mounting the magnet arrangements is further decreasedif the axial extension of the contact region in which the shank portionlies on the magnet arrangement is less than the axial extension of theside faces of the magnet arrangement.

It is also advantageous if the head portion lies on the peripheral facesin a contact region, which contact region is separated by a distancefrom a transition between the shank portion and the head portion. Thisresults in the magnet arrangements being reliably fixed to the rotorcore, further facilitating the mounting.

The distance from the head portion in the circumferential direction ispreferably 30% to 70%, particularly preferably 40% to 60% of the widthof the head portion from its free end to the transition to the shankportion.

The magnet holder can preferably be made in an injection mouldingprocess and inserted along with the shank portions into grooves of therotor core running in an axial direction before the magnet arrangementsare mounted on the rotor core.

A particularly advantageous embodiment provides for each magnetarrangement to have a permanent magnet and a magnetic field conductor,wherein the permanent magnet lies on an outer planar face of the rotorcore and the magnetic field conductor lies on the radial outside of thepermanent magnet. This increases the efficiency of the electric motorand decreases the weight and the rotational inertia moment. The magnetholder preferably only lies on the magnetic field conductor of themagnet arrangement. The magnetic field conductor can be made with asuitable surface which, together with the magnet holder, furtherfacilitates mounting.

The object is also achieved by a brushless electric motor having astator, a motor shaft rotatably mounted in a housing and a rotor unitwith the features and advantages described above mounted on the motorshaft. An electric motor of this type is particularly robust, easier tomanufacture and has a high degree of efficiency.

In the following, an embodiment of the invention is described in moredetail on the basis of the drawing. In the drawing:

FIG. 1: shows a planar view of a rotor unit according to the inventionin the direction of the central axis,

FIG. 2: shows a perspective view of the rotor unit from FIG. 1 withoutmagnets,

FIG. 3: shows a perspective view of an enlarged section of a rotor unitaccording to FIG. 1, and

FIG. 4: shows a cross section along the line A-A from FIG. 3.

FIG. 1 shows a planar view of a rotor unit 1 in the direction of acentral axis 2 which coincides with an intended rotational axis of therotor unit 1. The rotor unit 1 has an essentially rotationallysymmetrical rotor core 3 with a central bore 4 to receive a motor shaft(not shown) and a number of axially parallel bores 5. On the outside,the rotor core 1 has flat outer faces 6 and in this embodiment has atotal of eight outer faces 6, each of which is the same size and thesame shape and which outer faces 6 are distributed at an equal angulardistance along the outer peripheral face of the rotor core 3. A groove 7is provided between each two outer faces 6, which is formed from theoutside in the radial direction in the edge formed by the two adjacentouter faces 6 in this area. The groove is open in a radial directionoutwards and runs parallel to the central axis 2. A total of eightpermanent magnets 9 lie on the outer faces 6, which permanent magnetshave a planar contact face 10 and an outer circumferential face 11 andside faces 12 and 13. The side faces 12 and 13 are each arrangedpointing in the circumferential direction while the contact face 10points inwards in a radial direction towards the rotor core 3 and theperipheral face 11 points outwards in a radial direction away from therotor core 3. Finally, the permanent magnets 9 also have axial frontfaces 14, with only the front face turned towards the viewer visible inFIG. 1.

The permanent magnets 9 are held on the rotor core 3 by holding portions20 of a magnet holder 21 which is not fully visible here.

The holding portions 20 each have a shank portion 22 and a head portion23, wherein the shank portion 22 extends into the groove 7 and is heldthere in a positive-locking manner. The head portion 23 is moulded onthe end of the shank portion 22 opposite to the groove 7 and extends inthe circumferential direction of the arrangement. The permanent magnets9 are fixed by the holding portions 20 in the circumferential directionof the rotor unit 1 by lying on the respective adjacent shank portion 22with their side faces 12, 13. The permanent magnets are held by the headportions 23 in a radial direction.

FIG. 2 shows a perspective view of the rotor core from FIG. 1 butwithout the permanent magnets. The same components have the samereference numerals.

The outer faces 6 of the rotor core 3 can more easily be identified inFIG. 2. These are designed to be approximately square. The rotor core 3itself is made from a plurality of sheets of essentially the same crosssection. These are laminated to form a plate pack that forms the rotorcore 3.

The magnet holder 21 can be seen better in this view. It has a ringportion 24, the diameter of which approximately corresponds to thediameter of the rotor unit 1, which ring portion lies on the axial endface of the rotor core 3. The ring portion further comprises the totalof eight holding portions 20 which in FIG. 2 have been inserted into thegrooves 7 from below when mounting the magnet holder 21. The shankportions 22 extend in a radial, outward direction from the groove 7 andeach protrude outwards from the adjacent outer faces 6 of the rotor core3 in the circumferential direction. The head portions 23 are moulded onthe outer ends of the shank regions 22 and extend in an axial directionand in a circumferential direction.

FIG. 3 shows an enlarged, perspective view of a part of the rotor core 3with two holding portions 20 and a permanent magnet 9 insertedtherebetween. The holding portions 20 have a contact region 25 in theirshank portion 22, in which the adjacent permanent magnet 9 lies with itsside surface 12, 13 against the holding portion 20. The contact region25 is arranged in a radial direction at a distance from the outer face 6of the rotor core 3. A free space 26 is formed between the contactregion 25 and the outer face 6 in which the side face 12, 13 of thepermanent magnet 9 do not lie on the holding portion 20. The contactregion 25 has a height in the radial direction that corresponds toaround 50% of the height of the adjacent side face 12, 13. The radialoutside of the contact region 25 transitions directly into the headportion 23.

Finally, FIG. 4 shows a cross section along the line A-A from FIG. 3,again in a demolished and enlarged view. In contrast to FIG. 3, FIG. 4shows two permanent magnets 9 and a holding portion 20 lyingtherebetween.

The head portion 23 extends in a circumferential direction from thecontact region 25 of the shank portion 22, thereby protruding over theperipheral face 11 of the permanent magnet 9. A width c of the overlapof the transition of the head portion 23 into the shank portion 22 asfar as the free end of the head portion 23 is shown in FIG. 4 with thesegment length c. The head portion 23 does not lie on the peripheralface 11 along the entire width c but rather only lies on a partialportion, a contact area a which directly lies on the free end of thehead portion 23 and in the embodiment shown in FIG. 4 extendsapproximately 50% of the segment c. There is a distance b between thecontact area a and the transition from the head portion 23 into theshank portion 22 in which the head portion 23 does not lie on theperipheral face 11.

The lengths of the segments in the contact area a and the distance b inthe circumferential direction add up to form the extension of the widthc that is the above-mentioned overlap. The segment length a is thereforeapproximately 50% of the overlap.

Since, unlike in the prior art, the head portion does not lie on theperipheral face 11 along the full width, lower levels of friction occurin this region than in the prior art as described in the patentapplication US 2015/0001978 A1 when mounting the permanent magnets 9.Like the decreased friction in the contact region 25, this decreasedfriction results in the forces being decreased on mounting of thepermanent magnets 9. This prevents both wear and damage to the permanentmagnets 9 as described above.

Two different technical solutions to decrease friction when mounting themagnets 9 have been described. On the one hand, the contact face on theshank portion 22 has been decreased, and on the other hand the contactface a in the overlap c of the head portion 23 and the peripheral face11 is reduced. Each measure individually reduces the friction whichoccurs during mounting. Both measures are preferably used to achieve aparticularly good result.

The embodiment described above relates to rotor units with permanentmagnets 9. It can be generally used for rotor units which have magnetarrangements at the position of the permanent magnets 9 described whichcould, for example, be composed of permanent magnets and magneticconductors to shape the magnetic field in a suitable manner.

The invention claimed is:
 1. Rotor unit for a brushless electric motor,with an annular rotor core surrounding a central axis, a plurality ofmagnet arrangements which are arranged around the rotor core in acircumferential direction of the rotor unit, and which each have aconvex outer peripheral face, an inner contact face, two axial end facesand two side faces pointing in the circumferential direction, a magnetholder having a number of holding portions which are each arrangedbetween two adjacent magnet arrangements and moulded onto a ring portionof the magnet holder, wherein the holding portions are configuredT-shaped in a cross section along a plane running transversely to thecentral axis, and each have a shank portion and a head portion, whereinthe shank portion in a contact region lies on the side faces of themagnet arrangements and the head portion lies on the peripheral faces ofthe magnet arrangements, characterised in that the contact region of theshank portion in the radial direction of the side faces has an extensionof 30% to 70% of a radial extension of the side faces.
 2. Rotor unitaccording to claim 1, characterised in that the contact region of theshank portion in the radial direction of the side faces has an extensionof 40% to 60% of the radial extension of the side faces.
 3. Rotor unitaccording to claim 1, characterised in that the axial extension of thecontact region in which the shank portion lies on the magnet arrangementis smaller than the axial extension of the side faces of the magnetarrangement.
 4. Rotor unit according to claim 1, characterised in thatthe head portion lies on the peripheral faces in a contact area (a)which is separated by a distance (b) from a transition between the shankportion and the head portion.
 5. Rotor unit according to claim 4,characterised in that the distance (b) in the circumferential directionhas a width of 30% to 70% of a width (c) of the head portion from itsfree end to its transition to the shank portion.
 6. Rotor unit accordingto claim 4, characterised in that the distance (b) in thecircumferential direction has a width of 40% to 60% of the width (c) ofthe head portion from its free end to its transition to the shankportion.
 7. Rotor unit according to claim 1, characterised in that themagnet holder is made in an injection moulding process and inserted withthe shank portions in axially running grooves of the rotor core beforethe magnet arrangements are mounted on the rotor core.
 8. Rotor unitaccording to claim 1, characterised in that each magnet arrangementconsists of a permanent magnet.
 9. Rotor unit according to claim 1,characterised in that each magnet arrangement has a permanent magnet anda magnetic field conductor, wherein the permanent magnet lies on anouter flat surface of the rotor core and the magnetic field conductorlies on the radial outside of the permanent magnet.
 10. Rotor unitaccording to claim 9, characterised in that the magnet holder lies onlyon the magnetic field conductor of the magnet arrangement.
 11. Brushlesselectric motor with a stator, a motor shaft mounted rotatably in ahousing, and a rotor unit according claim 1 fixed to the motor shaft.